Spectral response measurements show that the photovoltage is produced primarily by strongly absorbed light. Surface photovoltage measurements have been made on cadmium sulfide crystals as disclosed by Richard Williams, Journal Physical Chemistry Solids, 23, 1057 (1962). FIG. 1 of the cited reference depicts a schematic drawing of a mounted crystal and circuit for measuring surface photovoltages. An ohmic contact of gallium-indium alloy is employed to make electrical contact with the crystal to serve as one electrode. A capacitive probe electrode, of transparent glass with conductive coating, is mounted so that this transparent probe electrode is separated from the crystal by an air space of 1 to 2 mils. This electrode and the ohmic contact is electrically connected to a Keithley electrometer amplifier having an input resistance of 10.sup.11 Ohms. The voltage signal obtained is either read directly or displayed on an oscilloscope.
The manufacture of silicon on sapphire (SOS) wafers has required that these wafers be evaluated at stages of processing and when finished for compliance with specification or for wafer preselection. This requirement has been partially met by the disclosed method by J. Lagowiski et al, J. Electrochem, Soc.: Solid-State Science and Technology, 128, Dec. 1981, p. 2665-2670. In this method surface photovoltage spectroscopy is applied to silicon-on-sapphire to enable a simultaneous determination of band structure, trapping centers, deep levels, the refracture index, and the film thickness. This method employs a monochromatic light (energy range 0.5-3.5 eV) which generates photovoltage that is measured with respect to a semitransparent gold electrode which is separated from the illuminated silicon layer of the SOS by a 10 m thick Mylar film or with respect to a semitransparent gold electrode vapor deposited directly on the silicon layer of the SOS.
The advancement of testing techniques for inspecting SOS wafers has been beneficial to the art; however, additional improvements to enable non-destructive testing of SOS wafers would be additionally beneficial.
Therefore, an object of this invention is to provide a device for use in a non-destructive testing method for inspection of silicon on sapphire wafers.
A further object of this invention is to provide a device for use in a non-destructive testing method wherein a capacitance is inserted in series in the measuring and calibration circuits of the device wherein the equivalent capacitance has no adverse effect.